Although I wouldn't take my word for granted (a similar thing happened to me where I tried to do irimi nage on someone with their back to the wall - it pinned them for a while but it wasn't very good 'cos they were twice the size of me) I would suggest:

a. remember that if you have committed to a technique and it doesn't work, change immediately. A ryote-dori kote-gaeshi, ikkyo, nikkyo or shiho nage may have been good if there was no movment to work with.
b. I find tenchi-nage a great warm-up technique. Practise it regularly with multiple attackers with nice flowing movements, then try to simulate the problem with a friend and your sensei so you can see the body mechanics.
c. to me visualisation is to help your body mechanics and focus - it won't work if you're not doing the technique right. Also the techniques in aikido are not so much hand here/foot there, it is about learning WHY your hand goes there and foot goes there so you can take someone's balance whatever the movement.

Obviously your own sensei would be better advised as he may be able to see where the problem lies. However you must make a distinction between play-fighting and aikido. With Aikido everything has to count and you should not hold back from striking forceful atemis and succeding at all costs.